This invention relates in general to vehicle brake systems and in particular to an algorithm for boosting the pressure in the rear circuits of such a brake system to provide enhanced braking when the vehicle is heavily loaded and/or towing a trailer.
Typically, vehicles are equipped with hydraulic brake systems having a dual master cylinder which includes separate front and rear hydraulic brake fluid reservoirs and actuating chambers. The front and rear actuating chambers are connected to front and rear brake cylinders, respectively, to define separate front and rear brake circuits. Depressing a brake pedal, which is connected by a mechanical linkage to the master cylinder, applies hydraulic pressure through both brake circuits to the brake cylinders at each of the vehicle wheels. The brake cylinders actuate the front and rear wheel brakes to slow the vehicle. By dividing the brake system into front and rear brake circuits, braking capability is maintained if a brake fluid leak should develop in one of the brake circuits.
Referring now to the drawings, a graph of rear brake pressure as a function of front brake pressure is shown in FIG. 1. Upon application of the brakes, equal pressure is applied to both front and rear brake circuits, as illustrated by the line labeled 2 in FIG. 1. The pressure increases linearly until maximum pressures are reached for the front and rear brake circuits, which are labeled PFM and PRM1, respectively. While an idealized linear relationship is shown in FIG. 1, it will be appreciated that a non-linear relationship may also exist between the front and rear brake circuit pressures (not shown).
During a braking cycle, a portion of the weight of a vehicle may be transferred from the rear vehicle wheels to the front vehicle wheels. The transfer of weight increases the frictional force produced between the front vehicle wheels and the road surface while decreasing the frictional force produced between the rear vehicle wheels and the road surface. Accordingly, if an equal braking force is applied to the front and rear vehicle wheels, the transfer of vehicle weight between the rear and front wheels could cause the rear wheels to lock up while the front wheels continue to rotate. A vehicle with locked rear wheels and rotating front wheels could spin-out easily.
To maintain directional stability during a braking cycle, the front wheels must lock up before the rear wheels. The use of brakes having a higher output on the front wheels results in a greater braking force being applied to the front wheels than the rear wheels when the same hydraulic pressure is applied to the front and rear brake cylinders.
On a low mu road surface, little or no weight is transferred from the rear to the front of a vehicle during a braking cycle. Thus, the higher output design of the front brakes is usually sufficient on low mu surfaces to assure that the front wheel brakes lock up before the rear wheel brakes, thereby preserving vehicle directional stability.
On high mu surface roads, however, the greater coefficient of friction of the road surface allows a harder brake application with a corresponding greater vehicle deceleration. The increase in vehicle deceleration results in a transfer of vehicle weight from the rear wheels to the front wheels. With the transfer of vehicle weight, the design of the brake calipers may not be sufficient to assure that the front brakes lock up before the rear brakes. Accordingly, a proportioning valve is typically included in the rear brake circuit.
The proportioning valve is operative to increase the hydraulic pressure applied to the rear wheel brake cylinders at a slower rate than the rate of increase of the hydraulic pressure applied to the front wheel brake cylinders, as illustrated by the line labeled 4 in FIG. 1. The proportioning valve is operative only after a predetermined brake pressure threshold, identified as PT in FIG. 1, has been exceeded. For hydraulic pressures below the threshold, the same hydraulic pressure is applied to both the front and rear brake cylinders. Also, the pressure applied to the rear brake circuit is limited to a maximum value, PRM2, which, as shown in FIG. 1, is less than the maximum front brake circuit pressure PFM. The different rates of increase result in a greater braking effort occurring at the front wheel brakes than at the rear wheel brakes such that the front wheel brakes lock up before the rear wheel brakes.
Currently, many vehicles are equipped with electronically controlled brake systems that include selectively controlled solenoid valves in the front and rear brake circuits to enhance control of the vehicle. Such systems include Anti-Lock Brake Systems (ABS), Traction Control Systems (TCS) and/or a Vehicle Stability Control Systems (VSC) and are typically actuated upon detection of a vehicle dynamic parameter exceeding a predetermined threshold. For example, an ABS is typically actuated upon detection of excessive wheel slip prior to the wheel locking up. Following actuation of the system, the solenoid valves are selectively operated to first isolate the affected vehicle wheel brakes from the vehicle master cylinder and then to control the hydraulic pressure applied to the wheel brakes to correct the problem. In place of a proportioning valve, an electronically controlled brake system may be utilized to proportion the pressure applied to the front and rear wheel brakes. One such electronic proportioning system is described in U.S. Pat. No. 6,357,836 where, above a predetermined pressure threshold, the pressure applied to the rear brake circuit is less than the pressure applied to the front brake circuit.
However, under certain conditions, such as when a vehicle is towing a trailer or is heavily loaded, as with a truck, the rear longitudinal tire force may not be fully utilized. Accordingly, it would be desirable, under such conditions, to apply a pressure to the rear brake circuit that is greater than the pressure applied to the front brake circuit in order to reduce the vehicle stopping distance.